Histograms act as graphical representations of the tonal distribution in digital images. Each histogram plots the number of pixels for each tonal value. Therefore, by looking at the histogram corresponding to a specific digital image, an individual can readily examine the entire tonal distribution at a glance.
Generally, the horizontal axis of a histogram represents the tonal variations, while the vertical axis represents the number of pixels in that particular tone. For example, the left side of the horizontal axis typically corresponds to darker tones (e.g., black), the middle of the horizontal axis typically corresponds to moderate tones (e.g., medium grey), and the right side of the horizontal axis typically corresponds to lighter tones (e.g., white) for monochromatic images (also referred to as “grayscale images”). Thus, a dark image will have the majority of its data points represented on the left side and center of its histogram. Conversely, a bright image will have the majority of its data points represented on the right side and center of its histogram.
Color histograms, meanwhile, represent the distribution of colors in a digital image. More specifically, color images can represent the number of pixels that have colors in each of a fixed list of color ranges that span the digital image's color space (i.e., the set of all possible colors). Although color histograms are often used for three-dimensional (3D) color spaces (e.g., RGB or HSV), color histograms can be built for any kind of color space. However, color histograms are often difficult to manage, particularly when the set of possible color values is large.